Added an EU Supply source, scraping the UK portal's public tenders

internal/eusupply replaces the old Node scraper (eu-supply.js) for
uk.eu-supply.com. The site has no API, so it posts the public search
form, which now needs the form's current defaults and reads the HTML
with golang.org/x/net/html. Each tender not stored yet has its own
page read too, for its description, contract type, CPV codes and
buyer address. Tenders are stored with the source "EU Supply - UK"
as eusupply-uk-<tender id>, with one release holding the tender as
read.

The site publishes no rate limit, so the client sets its own: one
request at a time, with a random wait between
sources.eu_supply.request_min_gap and request_max_gap (2s to 10s).
The source has the same 7-day backfill as Find a Tender's.

Most of its tenders are also on Find a Tender, so a new one is stored
Hidden and Skipped when another source's opportunity released within
30 days has a title at least 90% alike, and its buyer or description
matches too (80% of the shorter one's significant words).

internal/eusupply/convert.go

@@ -0,0 +1,101 @@
+package eusupply
+
+import (
+	"encoding/json"
+	"strings"
+
+	"git.avsdev.uk/Go/job-search/internal/types"
+)
+
+// The opportunities Convert makes.
+const (
+	// UUIDPrefix starts the UUID of every opportunity from the UK portal;
+	// the tender's id follows it.
+	UUIDPrefix = "eusupply-uk-"
+	// uuidType is their UUIDType.
+	uuidType = "eusupply"
+	// releaseTag is their one release's tag: the portal lists open tenders.
+	releaseTag = "tender"
+)
+
+// mainCategories maps the portal's types of contract to OCDS's
+// mainProcurementCategory, as Find a Tender's opportunities have.
+var mainCategories = map[string]string{
+	"Supplies": "goods",
+	"Services": "services",
+	"Works":    "works",
+}
+
+// UUID returns the UUID of the opportunity for the tender with id.
+func UUID(id string) string {
+	return UUIDPrefix + id
+}
+
+// ReleaseID returns the id of the one release of the opportunity for
+// tender: its id, as the portal gives each tender one page.
+func ReleaseID(tender Tender) string {
+	return tender.ID
+}
+
+// Convert returns the opportunity for each tender, with one release whose
+// JSON is the tender as read.
+func Convert(tenders []Tender) []*types.Opportunity {
+	opportunities := make([]*types.Opportunity, 0, len(tenders))
+	for _, tender := range tenders {
+		opportunities = append(opportunities, convertOne(tender))
+	}
+	return opportunities
+}
+
+// convertOne builds the opportunity for one tender.
+func convertOne(tender Tender) *types.Opportunity {
+	published := tender.Published
+	opportunity := &types.Opportunity{
+		UUID:              UUID(tender.ID),
+		UUIDType:          uuidType,
+		Title:             tender.Title,
+		Status:            "active",
+		Description:       tender.Description,
+		BuyerName:         tender.Buyer,
+		Classifications:   classifications(tender),
+		MainCategory:      mainCategories[tender.ContractType],
+		MultiLot:          tender.Lots > 1,
+		StartDate:         &published,
+		EndDate:           tender.Deadline,
+		Source:            Source,
+		LatestReleaseDate: tender.Published,
+	}
+
+	release := types.OpportunityRelease{
+		UUID:        opportunity.UUID,
+		ReleaseID:   ReleaseID(tender),
+		ReleaseDate: tender.Published,
+		Tags: []types.OpportunityReleaseTag{
+			{
+				Tag: releaseTag,
+			},
+		},
+	}
+	// A Tender is plain data, so marshalling it can't fail.
+	if encoded, err := json.Marshal(tender); err == nil {
+		release.JSON = string(encoded)
+	}
+	opportunity.Releases = []types.OpportunityRelease{release}
+
+	return opportunity
+}
+
+// classifications returns the tender's CPV codes as Find a Tender's are
+// stored: without the check digit ("48000000", not "48000000-8").
+func classifications(tender Tender) []types.ClassificationCode {
+	codes := make([]types.ClassificationCode, 0, len(tender.CPVCodes))
+	for _, cpv := range tender.CPVCodes {
+		code, _, _ := strings.Cut(cpv.Code, "-")
+		codes = append(codes, types.ClassificationCode{
+			IsCPV:          true,
+			Code:           code,
+			Classification: cpv.Description,
+		})
+	}
+	return codes
+}