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make github enterprise a checked constructor and keep the build cache out of history

Reconstructs the branch tip on d7560b54 without the .gocache and .gomodcache
trees that slipped into 61b2835 (about 10.7k files, ~400MB), and adds both to
.gitignore so they cannot come back. The scheme-less URL handling and the README
app-registration note from that commit are kept.

Reworks the enterprise support from the review:

An invalid base URL is now a registration error instead of a silent fallback to
public github.com. NewGithubEnterprise(p, baseURL) (Oauth2Handler, error) and an
error-returning AddGithubEnterpriseProvider replace the GithubEnterpriseURL field
on Params, so a mistyped URL fails at startup rather than quietly authenticating
people against the wrong server. The constructor takes Params, so the
UserAttributes and GithubNumericID combination needs no hand-built struct.

Seeds both id inputs with the instance authority, ghes:<realm>:login:<login> and
ghes:<realm>:gid:<id>, where the realm is the lowercase host plus any non-default
port. Public github.com ids stay byte-for-byte. Enterprise logins no longer
collide with their github.com namesakes, and http and https on one host resolve
to the same id. With no usable numeric id the enterprise-seeded login is kept,
not the public one.

Tightens the URL guard so a bare https://, a URL already carrying /api/v3, and a
trailing ? are all rejected: parse once, then build the root from scheme and host
instead of trusting u.String(). The invalid-url error carries none of the input,
and the one WARN that still names a URL goes through redirectHostForLog so a
credential-bearing value cannot leak.

Mirrored across the v1 and v2 modules.